Anal. Chem. 1994,66,445R-461R
Ultraviolet and Light Absorption Spectrometry J. A. Howell' Western Michigan University, Kalamazoo, Michigan 49008
L. G. Hargis University of New Orleans, New Orleans, Louisiana 70 148 Review Contents
Chemistry Metals Nonmetals Organic Compounds Simultaneous and Multiwavelength Determinations Derivative Determinations Reaction-Rate Determinations Flow Injection Determinations Photoacoustic and Thermal Lens Determinations Physics Optimization and Calculation of Results
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Precision, Accuracy, and Selectivity Standards and Calibration Stoichiometry and Physical Constants Algorithms and Software Instrument Components Spectrophotometers Specialized Instruments or Components
This review reports the developments in ultraviolet and light absorption spectrometry from January 1992 through December 1993,primarily as documented in the Ultraviolet & Visible Spectroscopy section of C A Selects, and extends the series of reviews on these topics sponsored by AnaZyticaZ Chemistry starting with Light Absorption Spectrometry in 1945 (2-3), followed by Ultraviolet Absorption Spectrometry in 1949 (4-9) and combined Ultraviolet and Light Absorption Spectrometry in 1978 (20, 2 2 ) . The format of this review follows that of its recent predecessors, with the subject matter divided into sections on chemistry, physics, and applications. The applications section is comprised of tables which summarize the routine determinations of inorganic and organic substances. The literature on ultraviolet and light absorption spectrometry continues to be so extensive and varied in scope that citations in this review are limited to those developments which the authors believe are of greatest interest to analytical chemists and those engaged in the chemical analysis of materials. As a result of this necessary selectivity, the authors apologize in advance for any errors of judgement made in the omission of specific citations. It is with great sadness that this year's review must report on the death of the founder of this review series and a giant in the development of analytical chemistry as a discipline, Melvin Guy Mellon. Professor Mellon died September 6, 1993,at the age of 99. He authored or coauthored with his former student, Professor David Boltz, the first 17 review articles on Light Absorption Spectrometry, starting in 1945. 0003-2700/94/0366-0445$14.00/0 0 1994 American Chemical Society
James A. Howell is a professor of Chemistry at Western Michigan University and alsoa science advisor for the Detroit District Laboratory of the Food and Drug Administration. He received his B.A. from Southern Illinois Universityin 1959, his M.S. from the University of Illinois in 1961, and his Ph.D. in analytical chemistry from Wayne State University in 1964. His particular fields of interest are in ultraviolet and visible absorptionspectrometry, flame emission and atomic absorption spectroscopy, and also computer applications to chemical instrumentation. He is the author of a number of research DaDers and chapters in books. Dr. Howeil is a member of the ACS, SAS, and the Association of Analytical Chemists. Larry G. Harglsis a Professor of Chemistry and Associate Vice Chancellor for Academic Affairs at the University of New Orleans. He graduated from Wayne State University with a B.S. in 1961, an M.S. in 1963, and a Ph.D. in 1964, spent a year as a Postdoctoral Research Associate at Purdue University, and joined the faculty at UNO in 1965. He has authored numerous papers on topics in ultraviolet and light absorption spectrometry, reaction-rate determinations, fast kinetics of analytical reactions, and heteropolymotybdate chemistry. He has also written two laboratory manuals, an introductory analytical chem- E istry text, and chapters in several monographs dealing with spectrophotometry. Dr. Hargis holds membership in the American Chemical Society (Analyticaland Educationdivisions), National Science Teachers Association, Phi Lambda Upsilon, and Sigma Xi.
Since 1978, the current authors, both of whom are former students of Professor Boltz, have attempted to produce reviews that would be judged to adhere to the high quality of those early reviews. So in effect, Professor Mellon has been directly or indirectly responsible for this series for 49 years. Review articles that are limited in scope to one of the categories of this review are cited in the appropriate section. Those of a more general nature or devoted Eo multiple topics are noted here. Articles of a general scope include a review of developments in UV-visible spectrometry in the past decade ( 2 2); an examination of numerous spectroscopic techniques, including absorptiometry (23);reviews of spectrophotometric analysis from September 1988 to June 1990 (24) and of the developmentof trace element determination over the past five years (25) in China; and the theory and uses of molecular spectroscopy ( 2 6 ) . A comparative study of chromogenic reagents and their applications has been reported (27) along with a number of other limited reviews on spectrophotometry including, applications in electrochemistry (18), solvent extraction proAnalyticalChemistry, Vol. 66, No. 12, June 15, 1994 445R
cedures (19),water analysis (20),and pulp and paper studies (21); determination of hazardous substances in biological materials (22);and principles of use in ecological studies (23). The number of books and chapters appearing during the two-year period is about the same as for the last few reviews. Two volumes of Organic Electronic Spectral Data have been published (24, 25). A new edition of Physical Methods in Chemistry contains chapters on Visible and Ultraviolet Spectrophotometry (26) and Absorption Spectroscopy of Transient Species (27)and the Encyclopedia of Experimental Chemistry has a chapter on Optical Measurements (28). A new volume of the Chemical Analysis series has appeared entitled Principles and Practices of Spectroscopic Calibration (29),and a published proceedings of the International Society of Optical Engineers reports on Optically Based Methods for Process Analysis (30).
CHEMISTRY This section deals with the chemistry involved in the development of suitable reagents, absorbing systems, and methods of determinations. After many years of declining activity in the area of inorganic chemistry, due primarily to the development of other competitive techniques such as inductively coupled plasma spectroscopy and X-ray fluorescence spectroscopy, the number of papers in this area seems to have stabilized. The number of papers devoted to organic constituents, however, continues to rise, especially in clinical, pharmaceutical, and general biological samples. As measured by the number of papers being published, interest remains high in multiple-wavelength, derivative, and flow injection techniques. Much of this is a direct consequence of the increased power and versatility of new instrumentation brought about by the incorporation of computers and microprocessors. Metals. Several reviews of methods and/or reagents for determining a metal or groups of metals have been published, including alkali metals (31), noble metals (32), rare earths (33),iron in human tissue and fluids ( 3 4 ,and toxic elements in soil (35). Recent applications of morin (36)and of various cationic dyes (37) to the determination of metals have been reviewed as well as the applications of spectrophotometry in rock and mineral analysis (38, 39). Numerous papers have appeared that report on the properties of chromophores used in the determination of metals. Four new hydrazones, 2-pyridinecarbaldehyde-3-nitro-2pyridylhydrazone, 2-pyridinecarbaldehyde-3,5-dinitro-2-pyridylhydrazone, 2-quinolinecarbaldehyde-5-nitro-2-pyridylhydrazone, and 6-phenanthridinecarbaldehyde-S-nitro-2pyridylhydrazone, have been synthesized and their properties and reactivities with various metal ions compared with those of 2-pyridinecarbaldehyde-S-nitro-2-pyridylhydrazone(40). The 3,s-dinitro derivative was successfully applied to the determination of nickel in steel samples. Two new triazoles, 3-mercapto-5-(2’,4’-dihydroxyphenylazo1’)- and 3-mercapto5-(3’,4’-dihydroxyphenylazo-1’)- 1,2,4-triazole, were synthesized and reported to form 1:1, 1:2, or 1:3 metal-to-ligand complexes with A13+,Hg2+, Bi3+,and a variety of transition metals, depending on the reagent and the solution conditions (41). Surfactants have been used to modify the reactions between heavy and noble metals and phenylazophenyleneazorhodamine derivatives, producing shifts of 70 nm in the 446R
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wavelength of maximum absorbance and significant increases in molar absorptivities ( 4 2 ) . Lanthanum, praseodymium, neodymium, samarium, gadolinium, and terbium have been reported to form 1:3 metal-to-ligand complexes with Nphenylcinnamohydroxamic acid that are quantitatively extractable into chloroform at pH 9.0 (43). Addition of xylenol orange to the chloroform extract increases the molar absorptivity of the complexes which exhibit maximum absorption between 590 and 610 nm. A few papers comparing or studying several reagents for a specific metal or group of metals have been published. 5-Bromophosphonazochromotropic acid was found to be superior to three other bromo derivatives for the determination of bismuth(II1) ( 4 4 ) . The molar absorptivity of the complex in perchloric acid was reported to be 9.4 X lo4 L mol-’ cm-1 at 635 nm and the calibration curve obeyed Beer’s law up to 1 bg/mL. In a study of reactions of group IIIB metals with o-phenanthroline derivatives and tetraiodofluorescein, the resulting 1:2:2 metal-to-ligand-to-ligand ternary complexes were found to exhibit maximum absorption in the pH range 5.8-9.4 and molar absorptivities of 1.44 X 104-l.25 X lo5 L mol-] cm-’ ( 4 5 ) . The complexation of vanadium(1V) with C1-, S03H-,AsOjHz-, and N02-substituted o,o’-dihydroxyazo compounds was examined, and the resulting 1:1 complexes were characterized in terms of their absorption maxima, molar absorptivities, and stability (46). A number of specific chemical systems have been studied, providing new insight into their use in spectrophotometric determinations. The optimal conditions for the determination of copper, zinc, cadmium, manganese, iron(III), cobalt, and nickel with 4-(2-pyridylazo)resorcinol in water and waterdimethylformamide mixtures have been established in regard to pH, reagent concentrations, and solvent composition (47). Sodium tetraborate buffer was recommended for the best results. The sensitivities for the determination of rare earths as 6-complexes with p-chlorophosphonazo have been shown to depend on their ionic radii, with cerium- or terbium-group rare earths in the presence of yttrium-group elements producing mixed complexes with enhanced absorptivities (48). The 1:4:2 Yb-reagent-Eu complex was reported to have a molar absorptivity for Eu of 2.0 X lo5 L mol-’ cm-I at 746 nm. A study of some as-triazines as chromophores for ruthenium has revealed that 3-(2-pyridyl)-5,6-diphenyl-as-triazineshortens the color development time, eliminates the necessity of a large excess of reagent, and increases the molar absorptivity of the colored tris-complex (49). N o appreciable differences were obtained in the formation constants for americium(II1) and curium(II1) complexes with fulvic acid determined by ultraviolet-visible absorption and time-resolved laser-fluorescence spectrophotometry (50). A potentially very useful procedure for stabilizing the widely used chromophore, dithizone, has been reported, which involves equilibrating the reagent with 6 M acetic acid in chloroform ( 5 1 ) . The mixture is said to show no signs of deterioration or reduced activity for up to 3 weeks. The positive interfering effect of some foreign ions on the spectrophotometric determination of mercury(I1) with 2-hydroxy- 1,4-naphthoquinone has been studied and evaluated using nonspecificity coefficients as a selectivity criterion (52). A thorough study of the extraction of platinum-group metals with various substituted dithiocar-
boxylic acids has led to the development of new methods for palladium and osmium that are selective and of comparatively high sensitivity (53). 0-Cyclodextrin has been reported to sensitize or enhance the absorbance of chrome azurine S and eriochrome cyanine R complexes of aluminum, gallium, and indium in the presence of cationic surfactants such as cetyltrimethylammonium bromide or cetylpyridinium bromide, but not in their absence (54). The sensitizing effect is said to result from the ability of the hydrophobic end of the surfactant to insert itself into the 8-cyclodextrin while the hydrophilic end is associated with the anionic metal-ligand complex. Among the new methods reported is the determination of titanium(1V) with thiocyanate and N-phenyl-N’-(Zchloropheny1)benzamidine after extraction into benzene (55).The selected amidine was reported to be the best of 10 substituted derivatives having molar absorptivities of 4.60 X 104-6.30 X lo4 L mol-’ cm-l at the absorption maxima of 420-425 nm and yielded a detection limit for titanium of 0.01 pg/mL. Other new methods include the determination of titanium and chromium in silica-based catalysts by converting them to titanium tetrachloride and tris(acety1acetone)chromium using gas-solid reactions followed by determination of titanium in acidic solution as its peroxy complex and of chromium in basic solution as the chromate ion (56);cobalt and iron(I1) with silica glass detectors doped with l-nitroso-2-naphthol and 1,lo-phenanthroline, respectively (57);and neodymium and promethium with 4-(2-pyridylazo)resorcinol and acetylpyridinium chloride (58). A method describing the determination of five lanthanides, La3+,Nd3+,Eu3+,Ho3+,and Yb3+,without separation has been reported using the absorbance at 240 nm as a measure of the total concentration and at absorption peaks in the 195-235-nm region for the individual metals after the addition of citrate and of DTPA complexing agents (59). The strongly absorbing ion-association complexes of nickel and mercury(I1) with aliphatic amines and fluorescein derivatives have been studied and new methods proposed for both ions (60). Methods for zinc and silver have been developed based on the interaction with dithizone, 2-pyridyl1-azonaphthol, and Michler’s thioketone immobilized on optically transparent cellulose membrane ultrafilters (61). The detection limits were reported to be 0.5 pg/mL. New reagents appearing since the last review include 242’benzimidazoly1azo)-5-N-carboxymethylaminobenzoic acid for cobalt, copper, nickel, vanadium, and zinc (62), 4-(2quinolylmethy1eneamino)- 1-phenyl-2,3-dimethyl-5-pyrazolone for copper and nickel (63),and pyrrole- and imidazo[ 1,2a]benzimidazole for thallium(II1) (64). 3-Bromobenzo-, 3-nitrobenzo-, 5-bromosalicyl-, and 5-nitrosalicylhydroxamic acids have been prepared and studied in regard to their reactions with inorganic ions (65), and two papers have reported on 3,5,6-tris-substituted 1,2,4-triazines as analytical reagents, the first describing the synthesis of 14 derivatives and the relative effect of the type and position of the substituent on the chromogenic properties of the reagent (66), and the second describing the properties and applications of 3-(6methyl-2-py)-5,6-bis(2-furyl)-as-triazinein the determination of copper (67). Nonmetals. The number of investigations focusing on inorganic nonmetals remains small. Reviews have appeared
on the use of Keggin-type heteropoly anions in spectrophotometric determinations (68); indirect methods for the determination of anions and certain organic compounds, especially those using ion- or ligand-exchange reactions (69); thedetermination of ozone in both gas and liquid phases (70); and the determination of nitrate in water (71). The unique characteristics of the ultraviolet spectrum of nitrous acid in acidic solution have been used to detect and determine as little as 3.0 X 1 V M nitrite in the presence of 0.1 M nitrate solution (72). Nitrate concentrations in foliage determined by direct ultraviolet absorption is reported to compare well with the results of ion-selective electrode methods (73). Two-, four-, and six-electron reduction products of 12-molybdoarsenate have been isolated as their potassium salts and, on examination, showed nostructural changes from the unreduced heteropoly anion (74). An indirect method for phosphates and phosphonates has been developed, based on the decrease in absorbance at 450 nm that occurs on reaction with a mixture of iron(II1) and azide ion in acidic solution (75). Lastly, a method for hydrogen peroxide has been reported, based on its ability to oxidize cobalt to a green product with a strong ultraviolet absorption (76). Organic Compounds. The determination of organic constituents, especially in pharmaceutical preparations and clinical samples, continues to grow and accounts for nearly half of the citations in the chemistry section of this review. A number of reviews of methods for the determination of specific substances or groups of substances have appeared, including drugs in the wastewater of pharmaceutical plants (77);drugs determined with 2,4,6-trinitrophenol(78);penicillins (79);ketones in blood (80);alcohols, phenols, ethers, and peroxides (81);the useof porphyrins for determining hydrogen peroxide, glucose, and tyrosine (82); chlorophylls (83) and pesticides and herbicides (84)by direct ultraviolet absorption; polymers and proteins using deconvolution techniques (85); lignin in solutions (86);and textile fibers in forensics (87). Other reviews include a general overview of the applications of spectroscopy in pharmaceutical analysis (88), the use of organic chromophores for organic substances (89), and a historical account of the spectrophotometric determination of hemoglobin (90). Reagents reported suitable for determining a family or group of related substances include tetrabutylammonium hydroxide in dimethylformamide for aromatic nitro compounds in pharmaceutical preparations (91),N,N-dimethylp-phenylenediamine for aldehydes in vegetable and fish oils (92), phenothiazine with N-bromosuccinimide for sulfonamides in pharmaceutical preparations ( 9 3 , N-bromophthalimide for N-substituted phenothiazines (94), 5-sulfosalicylic aldehyde plus iron( 111) for polyethylene polyamines (95),ninhydrin for alkaloids (Sa),molybdate for phospholipids in biological materials (97),Coomassie Brilliant Blue G-250 for glycated proteins (98),p-benzoquinone for total proteins in plasma (99), and chloranil for various amino compounds such as benzocaine, lignocaine, and reserpine (ZOO). The reactions of iodine monobromide with 28 phenols have been studied and the information was used to develop a general determination of phenols (101). The precision and sensitivity of the determinations were improved when iron(II1) was present as a catalyst. Another paper reports that various Analytical Chemistry, Vol. 66,No. 12, June 15, 1994
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phenols can be determined on the basis of their ability to reduce ferriin to highly colored ferroin, which can be measured after extraction into an organic solvent as an ion-association complex (102). Two quite different methods for determining hydroperoxides have appeared: one in which iodide ion is oxidized to highly colored triiodide ion in dilute acid using iron(II1) as a catalyst (103) and the other involving a reaction with glutathione peroxidase immobilized on N-(Zaminoethyl)3-aminopropyl glass (104). The glutathione activity diminished by the reaction with hydroperoxide was monitored through its reaction with 5,5’-dithiobis(2-nitrobenzoic acid). Simultaneous determination of alkyl sulfates and alkyl sulfonates as ion-association complexes with malachite green has been reported by the addition of nonionic surfactants to the solution which causes the appearance of two distinct absorption peaks, one for each type of analyte (105). Proteins and total amino acids have been determined simultaneously on the basis of the observation that the products of the reaction between p-benzoquinone and proteins exhibit an absorption band at about 350 nm, whereas for most amino acids the absorption band is around 440 nm (106). A difference method based on measuring the absorbance before and after reduction of the analytes with zinc metal in hydrochloric acid has been used to determined seven 1,4-benzodiazepines in pharmaceutical preparations (107). In another study, five 1,4benzodiazepines were determined by hydrolysis in acid followed by reaction of the resulting benzophenones with 3-methylbenzothiazolone hydrazone in the presence of ceric ammonium sulfate to yield red products (108). Despite the large diversity of methods and techniques for determining organic substances, comparative studies continue to be surprisingly few in number. One such study has reported that, on the basis of sensitivity and reproducibility, 4-amino2-chlorobenzoic acid, iobenzamic acid, and metachlopramide were the best of some 40 diazo reagents for determining estrogens (109). Iobenzamic acid yielded the largest molar absorptivity for natural estrogens. In another comparison study, 3,4,5,6-tetrachloro-2-carboxyphenylfluorone-manganese(I1) was found to be superior to numerous other dyemetal ion systems for the determination of proteins (110). After two independent comparisons of the Bradford Coomassie Brilliant Blue, Lowry alkaline copper, and Smith bicinchoninic acid methods for determinining proteins and an evaluation of the effects of sample type and standards used on the accuracy of the results, it remains unclear which assay is more accurate, but the Bradford method is faster, simpler, and less prone to interferences fromnon-proteins in thesample (111,112). After determining total hemoglobin and carboxyhemoglobin in 100 blood samples using five specialized, commercial spectrophotometers from IL, Corning, Radiometer, and AVL, and comparing the results with those obtained by gas chromatography and the manual cyanmethemoglobin method, no clinically significant differences were found between the manual and spectrophotometric methods (1 13). In general, all of the spectrophotometers underestimated the carboxyhemoglobin when present at concentrationsgreater than 2.5%. Numerous established methods have been studied for the purpose of making improvements in either accuracy, sensitivity, selectivity, or analysis time. The use of a citrate buffer at pH 4.6 reportedly enables free proline and various amino acids 440R
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to be determined accurately with phosphate-free ninhydrin in the presence of large concentrations of carbohydrates (114). Modifications of the 4-aminoantipyrine-potassium ferricyanide method for determining phenols have been proposed that reduce the volume of sample required without sacrificing any precision (115). In the determination of polyphenols by the Prussian blue method, phosphoric acid was found to be better that tetrasodium ethylenediamine or sodium hexametaphosphate as a complexing agent and gum acacia was reported to be superior to methocel60 HG, phosphomannan 2448, starch, or polysaccharide B as the protective colloid used to prevent precipitation (11 6 ) . The amount of methanol or acetone used in aqueous-organic solvents to extract tannins has been shown to affect the final measured absorbance ( 1 1 7 ) . Acetone is reported to react with vanillin, which is commonly found in the tannin samples, to produce a highly absorbing product that causes a substantial analysis error. The colorimetric method of Lowry and Tinsley, using sodium bis( 2-ethyl)hexylsulfosuccinate in isooctane has been applied to the determination of free fatty acids as a reversed micellar system (118). A methodof subtracting the absorbanceof interfering heme chromophores and bilirubin has been devised that makes it possible to determine lipids in blood plasma with 2-thiobarbituric acid (119). Bile acids heated in 78% sulfuric acid produce allylic and dienic carbocations that absorb at 290310 and 370-390 nm, respectively, and the absorbance at both wavelengths was found to be linear with concentration between 1 X and 5 X M (120). A study of the fructosamine method for determining glycoproteins has led to recommended conditions and procedures for sampling and storing blood samples, standardization, preparation of reagents, and developing the colored products (121). The Coomassie Brilliant Blue G method for determining proteins has been the subject of numerous studies, including the effectiveness of adding sodium dodecyl sulfate when used for urinary proteins (122),factors affecting the solubility of the absorbing species (123), and the nature and effect of the various charge forms of the reagent (124). Two papers have reported on the results of drug interference studies in the Coomassie Brilliant Blue G and 2,2’-bicinchoninic acid methods for proteins (125, 126). Although the Reissig method for the estimation of N-acetylamino sugars is often used as a specific method for N-acetyl-d-glucosamine, a recent study indicates that it is sensitive not only to the monomer but to all soluble oligomers as well (127). An ultraviolet diode-array detector with an autocleaning cell and sample line has been evaluated for the continuous monitoring of water quality (128). A diode-array detector has also been used in the identification of up to 10 different barbiturates by comparing their spectra with previously recorded spectra stored in a computer library (129). Among the new methods published since the last review are the determination of nitrosoureas in acidic solution with ceftizoxime (500 nm; 12.5-100 pg/mL) (130);@-adrenergic blocking drugs in bulk and tablets by conversion to dialkyl dithiocarbamates with carbon disulfide followed by complexation with copper(1) and extraction into chloroform (453 nm) (131);benzodiazepines, such as diazepam and lormetazepam, with N-bromosuccinimide (132); antihistamines, such as mequitazine and terfenadine, with either chloranilic acid, tetracyanoquinodimethane, or tetracyanoethylene as r-ac-
ceptors to give highly colored radical anions (540 nm; 750 and 843 nm; 400 nm, respectively) (133); hydroperoxy derivatives of linoleic acid with N-benzolylleucomethyleneblue in the presence of 15-lipoxygenase (660 nm; 0.5-10 1 s ) (134); and uronic acids from pectin using carbazole in 80% sulfuric acid with borate ions added (135). Two methods have been described for five guanidino drugs, one involving the formation of ion pairs with bromocresol purple a t pH 3.8 and extraction into chloroform (415 nm; 2.1 X 104-6.9 X lo4 L mol-' cm-l) and the other based on their reactions with iodine in chloroform (292 nm and 345 nm; 7.0 X lo3-2.4 X lo4 L mol-' cm-l) (136). Amino acid end groups in heparin have been determined by reaction with 2,4,6-trinitrobenzenesulfonicacid toformproductsabsorbingat 350and420nm (137). Another paper describes an indirect procedure for amino acids on solid supports by treatment with excess o-phthaldialdehyde and subsequentdetermination of the unreacted reagent with glycine (138). Direct ultraviolet measurement of piroxicam at 333 nm is said to be slightly more sensitive and reproducible than by ultraviolet difference measurements at 327 nm or HPLC using an ultraviolet detector at 254 nm (139). A method for conducting multicomponent determinations in drug formulations has been proposed using an ultraviolet diode-array spectrophotometer and full-spectrum calibration based on principal-component regression to simultaneously determine the concentration of the analytes and to account for interferencesdue to other sample components (140). No appreciable difference was found in determining the ratio of o- to p-toluenesulfonamide in mixtures by direct ultraviolet absorption and by HPLC with detection at 268 nm (141). A chargecoupled device camera system has been utilized for the online detection of proteins in agarose gel after electrophoresis and the results cpmpared well with the Coomassie Brilliant Blue staining method (142). Other papers reporting on reagents for protein and hemoglobin determinations include a reverse biuret method combined with copper(1)-bathocuproine (1 .O X 106 L mol-' cm-1) (143), o-sulfophenylfluoronetitanium(1V) for urinary proteins (144), 2-ketoglucose with hydrazine for glycated proteins (145), 3,3'-dimethylnaphthidene with hydrogen peroxide for hemoglobin (146), and phenol and o-cresol for glycated hemoglobins (147). Total bile acids in dried blood have been determined via a reaction with 1-methoxy-5-methylphenaziniummethyl sulfate catalyzed by 3a-hydroxysteroid dehydrogenase (148). A method for urobilinogen in feces, urine, bile, and serum based on oxidation followed by complexation with zinc in dimethyl sulfoxide is reported to be considerably more precise than other methods that rely on reduction of the analyte or reaction with Ehrlich's aldehyde (149). Certain spectral characteristics of buckministerfullerene in chlorobenzene have been reported (334 nm; 6.0 X lo4L mol-' cm-l) (150). In avery interesting study of the photochromic behavior of certain fulgides, it was discovered that 5-dimethylaminoindolylfulgideabsorbs in the diode-laser region at 780 nm, does not bleach upon irradiation with light, and forms a protonated species in trichloroacetic acid that does not absorb in the diode-laser region (151). An equilibrated mixture of the fulgide and its protonated analog can serve as a model of a rewritable photon-mode opticalmemory system using light at 403 nm for writing, 554 nm for erasing, and 780 nm for readout. A new technique has been
described for detecting organic species absorbed on graphite electrode surfaces using a vibrating grating to modulate the wavelength of incident radiation (152). SimuItaneous and Multiwavelength Determinations. Despite significant activity in this area, only two narrowly focused reviews have appeared: one on Kalman filtering (153) and the other on the determination of polluting compounds in petroleum pitch (154). Among the methods for inorganic substances are applications of a principal-component analysis partial least-squares calibration model to the determination of cobalt, nickel, and zinc with 2-(5-bromo-2-pyridylazo)-5(diethylamino)phenol(155) and a multiple linear regression analysis for the determination of binary, ternary, or quaternary mixtures of seven divalent transition metals with 4-(pyridyl2-azo)resorcinol (156). Zinc and mercury(I1) have been determined simultaneously as their complexes with 3 3 dibromo-PADAP in the presence of sodium lauryl sulfate using a double-wavelength standard-addition technique (157). Different mixtures of rare earths have been determined using several approaches, including a smooth accumulation derivative technique (158), a Kalman filtering technique using complexeswithp-bromoarsenazo (159),a free-floating factor analysis method with tribromoarsenazo- and tricyclazolecetylpyridinium bromide complexes (160), a Rosenbrock calculation procedure with DCB arsenazo and DCB arsenaze diethylenetriaminepentaacetic acid competitive coordination systems (161),an iterative target transformation factor method using tribromochlorophosphonazo complexes (162), and a three-wavelength calculation with ternary complexes of bromohexadecyltrimethylamine and azochlorophosphine( 11) (163). The carbon disulfide or carbon oxide sulfide released on acid hydrolysis of dithiocarbamate and tetramethylthiuram disulfide fungicides has been shown to react quantitatively with ethylenediamine, pyrrolidine, and morpholine, as well as previously reported piperidine to form ultraviolet-absorbing adducts that can be used for their simultaneousdeterminations (164). At the optimal conditions, all four reagents showed about the same sensitivity to both carbon disulfide and carbon oxide sulfide and a detection limit of 0.3 pg/mL. The simultaneous determination of nitrite and nitrate has been reported in two papers. In one, the absorbance at 219 nm was taken as an indication of total nitrogen and used in conjunction with the absorbances at 223 and 229 nm to calculate the individual concentrations (165). The values obtained by this method agreed well with those of the widely accepted naphthylethylenediamine method. The second method was based on the additivity of the molar absorptivities of the two compounds at given wavelengths and used absorbances at eight wavelengths in the range of 200-215 nm (166). The results were reported to be in good agreement with those obtained by an ion-chromatography method. The number of applicationsfor organic substancescontinues to remain large. The absorbance difference technique has been used to determine paracetamol in pharmaceutical formulations containing aspirin and caffeine (267 and 294.5 nm) (167), resorcinol in the presence of phenol (168), and chloramphenicol in ear drops (290 and 254.7 nm) (169). A dual-wavelengthstandard-addition method has been developed and applied to the determination of chlorpromazine in
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pharmaceutical preparations (17 0 )and hydrocortisone in the presence of ephedrine in ear drops (171). Mixtures analyzed by using simultaneous equations to calculate concentrations from absorption data at multiple wavelengths include vitamin A and related compounds in serum (172),captopril (6.0-14 pg/mL) and hydrochlorothiazide (3.0-7.0 pg/mL) from the amplitudes of second-derivative spectra (173), and m- and p-cresols (17 4 ) . A linear regression analysis of absorption data from a diode-array spectrophotometer using different calibration methods has been described and applied to the determination of mixtures of up to nine phenols ( 1 7 3 , and a simplex method has been developed for the identification and determination of individual substances in a complex mixture using a data base of the spectra of the pure substances (176). a-Naphthol and a-naphthylamine have been determined simultaneously in mixtures using a target factor analysis technique (177). It has been reported that by adjusting the concentration of the reference solution, multiple inflection points can occur which allow the simultaneous determination of phenylalanine, tryptophan, and tyrosine in aromatic amino acid mixtures (178). In other types of simultaneous determinations, the K-factor method has been applied to the determination of mixtures of sodium o-nitrophenolate, sodium p-nitrophenolate, and sodium 2-methoxy-hitrophenolate (179) and a least-squares calculation has been used for paracetamol, caffeine, and chlorpheniramine in pharmaceutical preparations (I80),sulfametrole and trimethoprim in drug tablets (181), and vitamins B1, B2, and B3 in tablets (182). The Kalman filtering technique continues to find applications such as the determination of synthetic food colorings (183, 184);a-naphthol, a-naphthylamine, 2,7-dihydroxynaphthalene, and 2,4-dimethylbenzaldehydein the concentration range (0-5) X 10" M (185);and caffeine, propylphenazone, and salicylamide in drug tablets (186). Several papers have reported on the use of simultaneous techniques with first- or second-derivative spectra including phenylephrine and carbinoxamine maleate in drug capsules and diphenhydramine and naphazoline in nasal jelly (187), methylene blue-hexamethylenetetraamine (273.0 and 221.3 nm) and methylene blueresorcinol (300.0 and 260.4 nm) mixtures (188), and chlorophenols as ion-association complexes with tetrabutylammonium nitrate extracted into chloroform (189). A firstderivative, absorption-shift method has been applied to the determination of methyl- and propylparaben (I 90), and phenols in water have been determined using a double extraction-concentration procedure and absorbance measurements with a diode-array detector (191). New developments include a discussion of the theoretical basis for carrying out multicomponent determinations with an external rather than internal standard (192). The method was evaluated with mixtures of psoralen and furocumarin using papaverine as the external standard. The practical application of several multivariate calibration routines has been evaluated using a transition-metal-ion model system (193). A new method called spectral K-ratio spectrophotometry has been described which uses the K-coefficient of a dual-wavelength method in mathematical calculations that resolve the different nth derivative spectral data at one wavelength (194). The basic principles of combination differential amendment matrix spectrophotometry have been 45QR
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described and applied to the simultaneous determination of vitamins B1 and Bg, and oryzanol in drug tablets (195). Derivative Determinations. The general theory and use of first-derivative techniques (196) and the applications of derivative spectrophotometry in clinical analysis (1 97) and in drug analysis (198, 199) have been reviewed. Among the methods for inorganic substances that have appeared are the use of first- and second-derivative spectra for determining iron(II1) with EDTA in the presence of chromium(III), aluminum, and manganese(II1) (200) and the use of firstderivative spectra for determining simultaneously nickel, cobalt, and vanadium (201) and six rare earths (202). The use of third-derivative spectra was reported to eliminate interferences from other metals and improve the sensitivity for copper in iron and aluminum alloys (0.64-3.2 pg/mL; 0.1 5 pg/mL) (203). A second-derivative technique has been applied to the determination of nitrate in soils (204). The determination of organic substances, especially drugs, continues to be the most active area of derivative spectroscopy. Papers reporting on single-constituent determinations include the following (derivative order; wavelength used; linear range of calibration plot): atropine sulfate, homotropine hydrobromide, tolazoline hydrochloride, tetrahydrozoline hydrochloride, and xylometazoline hydrochloride after reaction with picric acid and extraction into chloroform (first; 420 nm; 2.010.0 pg/mL) (205); benzalkonium chloride (second; 260275 nm; (0-1 -5) X w/v W ) (206);caffeine (second) (207209);clotiazepam (first) (210);cytarabine (second; 299.4 nm; 1 X 10-5-5 X L mol-' cm-l) (211);imipenem with 4-(2hydroxyethy1)piperazine-l-ethanesulfonic acid (third; 3063 12 nm) (212); a-phenylethlyamine (second; 248-250 nm; 15-160 pg/mL) (213);potassium guaiacolsulfonate (third) (214);and salbutamol sulfate (second; 1-80 pg/mL) (215). Papers reporting on simultaneous determinations include acetaminophen plus orphenadrine citrate, ibuprofen, or chlorzoxazone (second) (216 ) ; amoxycillin and dicloxacillin (second; 308.5 nm, 275 nm; 20-200 pg/mL, 20-140 pg/mL) (217 ) ; caffeine, theobromine, and theophylline (first and second) (218); enalapril maleate and hydrochlorothiazide (second) (219); ephedrine hydrochloride and nikethamide (second; 256.7 nm, 26 1.1nm) (220);notriptyline hydrochloride and perphenazine (fourth; 239.6 nm, 268.8 nm) (221); paracetamol and mephenoxalone (third; 239.4 nm, 249.8 nm; 5-20 pg/mL, 5-1 3 pg/mL) (222); salicyladehyde, 3-hydroxybenzaldehyde, and 4-hydroxybenzaldehyde (first) (223); sulfathiazole and sulfametazine (fourth; 0.20-12.0 pg/mL) (224); and vitamins B1 and Bg (first; 286.1 nm, 268.5 nm) (225). A first-derivative difference method has been described for the determination of four thioxanthene drugs based on measuring the first-derivative spectrum of the oxidized drug relative to a solution of unoxidized drug (226). Two articles have reported on the simultaneous determination of chlorophenols using first-derivative spectra recorded with a diodearray instrument (227,228). Simultaneous determination of pesticides have been accomplished by first-derivative (229) and second-derivative (230)techniques. Benzene and nicotine in ethanol extracts of tobacco have been determined simultaneously by first-derivative measurements (231). Among the papers reporting on simultaneous determinations in clinical samples are carboxyhemoglobin and total hemoglobin (third)
(232), paraquat and diquat in plasma after reduction with alkaline sodium dithionate (second; 454.3-446.3 nm; 396460nm) (233),and metalloporphyrins (third) (234). In other applications, various mixtures of five synthetic food colorings have been determined simultaneously (first) (235) and p-phthalic acid has been measured in wastewater (first; 240 nm; 0.05-10.0 pg/mL) (236). The effect of multiple interacting factors on precision in ultraviolet derivative spectrometry has been studied and shown to be of considerable importance (237). The Fast Hartley transform technique has been used to reduce significantly the time required to identify drugs by matching their derivative spectra with those from a stored library (238). It has been reported that theuse of ratios of the derivative maxima permits an exact compensation of the effect of either component in a binary mixture, which leads to improved precision and accuracy in the simultaneous determination (239). Another study describes a damping factor matrix approach to analyzing overlapping derivative spectra (240), and noise in some derivative spectra has been attributed to a positive bias resulting from a peak-finding algorithm (241). Reaction-Rate Determinations. Two differential simultaneous methods have been reported, one for binary or ternary mixtures of rare earths based on the rate of displacement from their truns- 1,2-~yclohexanediaminetetraacetic acid complexes by arsenazo I11 (242) and the other for mixtures of cobalt and nickel based on the rate of displacement from their EGTA complexes with 4-(2’-pyridy1azo)resorcinol (243). Plasma and urinary hemoglobin have been determined at concentrations as low as 3 pg/mL by following the oxidation ofchloropromazineat 528 nm (244). It has beendemonstrated that reactions involving no suitably absorbing species, such as the acid-catalyzed hydrolysis of aliphatic alcohols, can still be monitored by addition of a small amount of an “assistant” reagent like chromium trioxide (245) and a new method has been proposed for the kinetic analysis of reactions with two linearly independent concentration variables (246). Rare earths in human hair have been determined at concentrations up to 0.02 pg/mL on the basis of their catalysis of the discoloring reaction of pyrocatechol violet by hydrogen peroxide (247). The reaction of straight-chain alcohols with an alcohol dehydrogenaseNAD system, monitored by measuring the initial rate of change of absorbance of the reduced coenzyme formed using a stopped-flow technique, has been used todeterminesimplealcoholsat thenanogram level (248). A new computational approach for the simultaneous determination of catalysts and activators has been reported in which the parameters and variables are optimized from experimental data obtained from either a multiple standard addition method or the Gauss-Newton and simplex method (249). Three continuous-flow approaches to the development of reactionrate methods have been proposed that are based on the use of immobilized enzymes as catalysts (250). Flow Injection Determinations. The use of this general technique for moiiitoring multiple components in process streams (251) and recent advances in injection systems and flow controllers (252) have been reviewed. The potential of a zone-sampling system comprised of a diode-array detector in conjunction with a variable-volume, dual-channel bypass
injection valve was studied and illustrated with the simultaneous determination of copper and zinc using 4-(2-pyridylazo)resorcinol (253). This same reagent was used for the determination of terbium, holmium, and europium at 5 15 nm in a dual-line flow system (254). Another paper reports on the determination of the lanthanides using chlorophosphonazo 111, in the 0-40 pg/mL concentration range, in the effluent of a cation-exchange column (255). A system employing a two-line manifold, one line for aqueous boron samples and the other for buffered chromotropic acid reagent, was reported suitable for determining 40-6000 pg/mL boron (256). The sensitivity for determining sulfate in lithium carbonate using barium chloridecetyltrimethylammoniumbromide as the carrier solution has been greatly improved through the use of a double-channel synchronous rotating-injector valve which produces two samples divided by a section of reagent in the flow stream (257). A flow injection stopped-flow analyzer has been constructed and evaluated for the determination of ammonia in the 20-250 pg/mL range using the Berthelot reaction, of creatinine in the 20-220 pg/mL range using the Jaffe reaction, and of phosphate in the 5-30 pg/mL range using a heteropoly blue reaction (258). The greatest utility of the flow injection technique appears to be in the determination of organic compounds as evidenced by the number of papers that have appeared, including free and total cholesterol after enzymatic oxidation and reaction of the liberated hydrogen peroxide with ammonium 2,2’azinobis-3-ethylbenzothiazoline-6-sulfonate (425 nm; 0.5-0.8 mmol/L) (259),various cardiac glycosidesafter reaction with picric acid (486 nm; 1+200 pg/mL) (260), acetylcholine and choline after extraction and formation of ion-association complexes with tetrabromophenolphthalein ethyl ester (6.3 X 10-’-7.5 X 10“ M, 1.3 X lv-1.5 X l t 5 M) (261), phenothiazines after reaction with either potassium iodate or ferric chloride (262), tetracyclines with iron(II1) (263), reducing sugars in wine, honey, and syrups using the copper(11)-neocuproine reaction (264), acetic acid in wine after diffusion into a stream of bromocresol purple (265),and the total polyphenol index in wines based on the Folin-Ciocalteu reaction (266). Proteins in serum have been determined in theconcentration rangeof0.01-0.1 ,ug/mL by direct injection into a stream of beryllium chromazurol B complex (267) and no significant difference in results was found between this method and the pyrogallol red-molybdenum(V1) spectrophotometric method. The peroxyoxalate chemiluminescence of eosin Y with bis(2,4-dinitrophenyI) oxalate and hydrogen peroxide was found to be markedly enhanced by bovine serum albumin and was used as the basis for both a direct (1.0 X 10+-1.0 X 1 t 2pg/L) and an indirect (6.6 X 10-3-3.3 X pg/L) determination of proteins (268). The use of neutral dyes such as rhodamine B and 4- [4-(dimethy1amino)phenylazo]-2-methylquinoline was found to be superior in terms of sensitivity to cationic dyes such as methylene blue and ethyl violet for the determination of anionic surfactants after extraction (269). As reagents for determining phenol, diazotized p-nitroaniline and 4-aminoantipyrine produced about the same results (270). Amino acids have been determined by their ability to leach entrapped cupric ions from a polymeric matrix and carry them into a packed-bed reactor where they react with zincon to produce a blue product Analytical Chemistry, Vol. 66, No. 12, June 15, 1994
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absorbing at 600 nm (271). The calibration plot for glycine was linear over the range 0.5-20 pg/mL. Among the special applications and new techniques of flow injection analysis that have been reported is the use of long path length flow cells to improve detection limits (272). In a study of the characteristics of knitted open-tube figure-eight (KOT) and coil reactors, it was determined that peak broadening was independent of flow rate for the KOT but not the coil reactors at flow rates of 150-300pL/min (273).Below 150 pL/min, the plate height values of both reactors were inversely proportional to flow rate. An experimental factorial design has been proposed for studying the effects of flow rate, injection volume, and coil length on the precision of flow injection signals (274). The advantages and limitations of detector location in flow injection systems have been discussed, specifically in regard to the issues of dead volume, manifold dimensions, and commutation timing (275). A small, dualwavelength, light-emitting-diode-based detector has been developed that uses no mirrors or lenses and has nearly identical paths for both beams (276). Rapid modulation of the light beams enabled the dark current and both beam intensities to be measured almost simultaneously, which dramatically reduced the effects of drift. From a study of the wavelet transform in the determination of peak intensities, it has been suggested that, for white noise and a favorable peak shape, a signal-to-noise ratio of 2 can be tolerated at the 5% error level, which means that a significant reduction in the detection limit is possible compared with classical signal-processing methods (277). Lastly, a laboratory exercise has been described making use of a modified Bertholet reaction to demonstrate the relationship between reaction kinetics and sensitivity when the analytical signal is measured under conditions where chemical equilibrium is not attained (278). Photoacoustic and Thermal Lens Determinations. There have been a significant number of reviews of thegeneral aspects of thermal lens spectrometry (279-284). A chapter in the new edition of the Physical Methods in Chemistry series is devoted to photoacoustic and photothermal techniques (285). The principles and specialized applications of photoacoustic spectroscopy have been reviewed (286)as well as applications in environmental trace gas analysis (287). A systematic methodological approach to adapting conventional spectrophotometric methods to thermal lens methods has been described and illustrated with the determination of cobalt with 2-nitroso- 1-naphthol, nickel with a-dioximes, molybdenum with thiocyanate ion, iron(I1) with 1,lO-phenanthroline, and bismuth with iodide ion (288). The feasibility of speciation of actinides in solution and solid phases has been studied with Fourier transform laser-induced and ordinary ultravioletvisible-near-infrared photoacoustic spectroscopy (289). A thermal lens spectrometry detector has been evaluated for the flow injection determination of terbium with 4-(2-pyridylazo)resorcinol (290). A photoacoustic method for ammonia using a tunable carbon dioxide laser with a windowless resonant cell compared favorably with the well-known colorimetric indophenol blue method (291). In another experiment, the same instrument and cell was able to detect as little as 70 ppb of ethylene produced by ultraviolet radiation of different types of polymeric tubing. A laboratory-constructed thermal lens 452R
AnalyticalChemistry, Vol. 66,No. 12, June 15, 1994
spectrometer has been used to determine ultratrace concentrations of theophylline (292). In a comparison of conventional and thermal lens techniques for measuring the absorbance of samples that scatter radiant energy, it was determined that the large errors occurring in conventional transmission-based measurements are absent in thermal lens measurements (293). New instrument components include a photoacoustic fiber-optic pressure sensor, comprised of a coiled optical fiber with a frequency-doubled Nd:YAG laser installed in the sample cell compartment, that measures the decrease in intensity of a helium-neon laser beam passing through the fiber (294) and a computercontrolled pulsed-flow system where the flow pulses are synchronized with the laser-pumping and data acquisition periods (295). A new technique of obliquely crossed and differential thermal lens spectrometry has been described and shown to be able to significantly reduce solvent effects in routine measurements (296). The photoreduction of 12molybdophosphoric acid in butanol, isobutyl alcohol, and tertbutyl alcohol has been studied (297)as has the effect of various optical and geometrical parameters of powder samples and of the spectrometer cell on the measured photoacoustic signal (298). The heat conversion efficiencies of photoexcited molecules in solution at room temperature were determined using a time-resolved thermal-lensing technique, and several substances were suggested as good calorimetric standards (299).
PHYSICS This portion of the review deals with topics that are related to the measuring of radiant energy, the treatment of data, and the instrumentation used in light absorption and ultraviolet spectrometry. Optimization and Calculation of Results. A comparison of the integral absorption characteristics of mixtures of organic substances has been reported to be useful for evaluating the degree of similarity or difference of their composition and for providing sufficient information on the nature of the constituents to make their identification possible (300). A simple method for interpretation of a set of absorption spectra based on rank determination of the data matrix has been developed that allows one to find the independent spectra from which all the others can be computed using a method of linear combinations (301). Methods for calculating the optimal working range (302) and optimal wavelengths for multicomponent determinations (303)have been reported and the factors affecting them have been discussed. An equation has been developed that enables the optimal time interval to be calculated from the rate constant for the fixed-time kinetics technique with pseudo-first-order reactions (304). Errors. The definition, origin, and effects of stray light on spectrophotometric measurements have been reviewed (305). A model has been proposed for more accurate estimation of errors based on covariance of calibration curve slopes and intercepts, the vector space concept, and a Laplace operation (306). Another paper, dealing with partial leastsquares (PLS) regression that is widely used in chemometrics, describes a means of linearizing the PLS estimator to allow the construction of approximate confidence intervals for predictions from PLS, thereby avoiding computationally
intensive bootstrapping procedures (307). The capability of the standard-addition technique to correct for matrix effects and the advantages of two-dimensional bilinear analytical data were used in the development of a hybrid method that reportedly compensates for both matrix effects and the influence of unexpected interferents in multivariate calibration (308). The absorbance of cresol red solutions at 575 nm in tris(hydroxymethy1)aminomethane buffer was determined to be very sensitive to changes in temperature, making it useful for monitoring temperature equilibration times and temperature stability in spectrophotometers and specialized analyzers (309). Precision, Accuracy, and Selectivity. Two strategies have been described for validating photometric and wavelength accuracies, spectroscopic bandwidths, and signal-to-noise ratios in ultraviolet spectrometers (310). A collaborative study of the reliability of using ultraviolet spectrometry for quality control in the manufacture of drugs, involving 32 laboratories, indicated that sample preparation and calibration of the instruments were primarily responsible for the observed differences (311). A model to calculate sensitivity, detection limit, and precision of a method using data obtained from calibration experiments has been described and reported to compare well with the model suggested by IUPAC (312). Standards and Calibration. Reviews that have appeared on this topic include the calibrationof instruments (313,314), principles of calibration (319, and theevaluation of calibration curves (316). Five optical transmittance standard reference materials from the National Institute of Standards and Technology, for monitoring stability and for verifying the accuracy of the absorbance or transmittance scale of spectrophotometers, have been described (317). A wavelength calibration procedure using two well-separated monochromatic spectral lines has been developed for low-resolution photodiode array spectrometers (318). Calibration techniques that have been reported for multicomponent systems include ridge trace analysis, which has been applied to the simultaneous determination of drug mixtures from their ultraviolet spectra, (319) and a modified stepwise regression method (320). A statistic Fk test has been proposed for detecting several outliers in linear regression analysis of data (321), and a protocol has been reported for standardizing calibration models between different instruments (322). Stoichiometry and Physical Constants. A method for calculating the stepwise stability constants of complexes has been reported that uses the characteristics of isosbestic points appearing in the spectra of a series of solutions to resolve the overlapping stepwise equilibria into single equilibria which are then processed by an equilibrium-shift method (323). Another paper describes a method using the two isosbestic points of a dual-ligand, single-metal system to determine the equilibrium constants of complexes in which two ligands compete for the same metal and one complex does not absorb at the measurement wavelength (324). Glenn’s method of orthoganol functions has been extended to the determination of dissociation constants of some weakly acidic and basic drugs (325). It has been reported that highly accurate pH estimations can be made conveniently by evaluating the spectra of an acid-base indicator suitable for the pH range of interest and whose pKa is accurately known (326). The method is
equally applicable to determining pKa values for indicators from accurately measured pH values. The dissociation constants of several weak acids and complexes have been determined, including 2,4-dinitrophenol herbicides such as dinoseb, dinocap, binapacryl, dinobuton, and dinoterb (327); daidzein (328);and europium(II1) murexide (329). A targettransformation factor-analysis method has been used to determine simultaneously the valence states of chromium and iron in a mixture of their complexes with 2-(5-bromo-2pyridylazo)-5-(diethylamino)phenol(330). A very nice laboratory experiment has been described that involves determination of the enzyme activity of alcohol dehydrogenase using the change in absorbance with time at 340 nm to monitor the rate of reduction of NAD+ by ethanol and the concentration of NADH when thereaction reaches equilibrium (331).From these data, the equilibrium constant for the reaction can be calculated. Algorithms andSoftware. The principles and use of expert systems in general and in analytical chemistry (332)and recent developments in nonlinear multivariate calibration (333) have been reviewed. The H-point standard-addition method has been modified to permit the determination of unbiased analyte concentrations in the presence of an interferant (334). The method, referred to as K-ratio H-point standard additions, uses the absorbances at two selected wavelengths and the K-ratio of an interferant at the same wavelength to give two straight lines with a common point H whose coordinates are the concentration of the analyte and the absorbance of the interferant at one of the two wavelengths. Algorithms making use of the full spectra for simultaneous multicomponent determinations are reported to have considerable advantages over conventional mathematical techniques (335). The deconvolution of overlapping spectra of multiple analytes has been accomplished by a minimization algorithm (336) and fuzzy linear programming (337). Algorithms that have been used for the simultaneous determination of multiple analytes include a K-factor method for both three (338) and four (339) components; partial least squares for active ingredients in ophthalmic solutions ( 3 4 4 , substituted naphthalenes (341) and general pharmaceuticals (342); factor analysis for food dyes ( 3 4 4 , iron and copper complexes with 1,lO-phenanthroline and 2,9-dimethyl- 1,10-phenanthroline, respectively (344), and amino acid drugs (345);artificial neural networks for five-component mixtures (346) and for mixtures of B vitamins (347); principal-component analysis for cobalt, copper, and zinc as their 4-(2-pyridylazo)resorcinol complexes (348); Kalman filtering for amino acids (349);orthogonal function spectrophotometry for amino acids (350);and vector/ matrix calculations for mixtures of rare earths (351). Another group of papers has reported on some other aspects of multivariate analysis, including a fast algorithm for identifying the qualitative composition of a sample (352); a review and critical comparison of full-spectrum standard least-squares, partial least-squares, and principal-component regression techniques (353); a robust procedure called sine function M-estimator (354); expert systems (355); comparison of quantification IV, cluster analysis, and principal-component analysis (356);comparison of classical least-squares, inverse least-squares, principal-component, and partial least-squares regression for drug mixtures (357); and an algorithm named Analytical Chemistry, Vol. 66, No. 12, June 15, 1994
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MULTI that deconvolutes standard spectra into contributions of constituents for storage and later comparison with unknowns (358). The need for a conventional calibration curve has been eliminated and replaced by a fuzzy linear regression method that uses an estimation area from which the concentration of unknown can be determined (359). A low-pass Chebyshev type I1 digital filter was found to be the filter of choice for signal processing in flow injection measurements because it employs the largest number of optimizing parameters (360). A Monte Carlo simulation has been applied to the study of error propagation in multiple standard-addition spectrometry (361). The practical difficulty of determining two components simultaneously when there is a large difference in their concentrations or molar absorptivities has been overcome by using two cuvets of different optical path length with an appropriate mathematical algorithm for calculating concentrations (362). Instrument Components. Reviews on the development and applications of fiber-optic sensors (363) and on selective photodetectors for the spectral range of 0.1-50 pm (364)have appeared. A theoretical analysis of the difference in absorbance measurements with conventional cells and cells with multiple reflections on the walls parallel to the optical beam has shown that the gain in absorbance is quite small and the effect on the linearity of the calibration plots is negligible (365). A new spectroelectrochemical cell has been described and yielded a measured diffusion coefficient for K4Fe(CN)6 that was identical with the value reported in the literature (366). A simple optical correction system has been devised that allows cylindrical vials to be used as sample cells without introducing significant error (367). Samples as small as 2 p L can be measured simply and accurately in a new cell that is suitable for use in commercial spectrophotometers, provided they have high energy throughput and highly focused beam optics (368). The construction of two high-temperature cells is reported, one suitable for temperatures up to 473 K (369) and the other consisting of two sample chambers arranged in sequence (370). A fountain cell, constructed for flow injection analysis, exploits radial flow to produce a thin disk-shaped detection volume with a large surface area (371). Two sandwich-type fiber-optic flow-through cells were compared in regard to their performance for absorption detection (372), and new cell designs for specialized applications of photoacoustic spectrometry continue to be published (373-375). An optical interface of a fiber-optic probe to a diode-array detector has been developed for drug dissolution testing (376) and simple, inexpensive, high-performance, optical absorption detectors have been devised for light-emitting-diode radiation sources (377). Two papers have reported on the useof chargecoupled devices as detectors (378,379). The sources of noise in photomultiplier tube detectors and in ultraviolet, visible, and near-infrared interferograms acquired with silicon and germanium diodes were studied using very similar conditions with each detector so that comparisons could be made with as little bias as possible (380). Spectrophotometers. The commercial spectrophotometers exhibited at Pittcon '92 (381,382)and Pittcon '93 (383,384) have been reviewed. Other reviews have appeared on landmark instruments that have been developed in the past 50 years (385),the system structure and principles of spectrophotom454R
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eters (386, 387), multichannel instruments with echelle gratings (388),and fiber-optic spectrophotometers (389).No major innovations in commercial instruments have been reported but significant improvements in stability, versatility, and data processing continue to be common. Acton Research has marketed the SpectraPro i series of imaging spectrographs designed for use with charge-coupled device detection systems and featuring triple indexable 275- and 500-nm focal length gratings. Two new instruments from Alton have been announced: the Polychrom PX-10, which uses a dual light source and photodiode charge-coupled device array detector and the Lamda LS-2000, a conventional analyzer operating at a 500-kHz scan rate with a readout time of about 5 ms per scan. Ametek has developed a new microprocessor-based control unit for their 4000 series UV-visible photometric analyzers. A spectroradiometer featuring a 5 12-element plasma-coupled photodiode array detector, suitable for use in the 350-1050-nm spectral range with a 3-nm spectral resolution, has been introduced. Beckman has added a new instrument to its DU-60 series, the DU-68, which combines all the power and versatility of the basic DU-65 but includes a video display that allows for simultaneous data acquisition and viewing in real time. The company has also unveiled a new line of spectrophotometers, the DU-600 series, which offers microvolume sample measurements, scan speeds up to 2400 nm/min, simultaneous time-based kinetics analysis of up to 12 reactions, and powerful Windows-based software. An instrument for measuring the color of powders, granules, solid materials, and opaque liquids has been marketed by BYKGardner, and Datacolor has introduced the Dataflash 200M spectrocolorimeter designed for measuring metallic, pearlescent, and solid colors for the automotive and paint industries. GBC has added to its line of spectrophotometers with the Models 918 and 920. Both instruments are dual beam with very low stray light and photometric noise. The Model 920 features a full double-monochromator optical system. The ProSpec series of fiber-optic spectrophotometers for process control in the UV, visible, and near-IR regions has been developed by Guided Wave. The instruments use a singlestrand fiber-optic cable to interface to the sample. Hewlett Packard has upgraded its Model 8452 diode-array instrument to the Model 8452Win. In conjunction with the company's ChemStation, this model has greatly expanded capabilities in both method development and research. Hitachi has added toits3000-linewith theModelsU-3000,U-3300,andU-3501. The single monochromator U-3000 and the double monochromator U-3300 share the same excellent specifications except for the lower stray light of the U-3300. The U-3501 is a UV-visible-near-IR instrument using a cooled lead sulfide detector for superior signal-to-noise ratio throughout the nearIR region. Three new models have been introduced by Jasco: the V-550, a double-beam, single-monochromator instrument for the UV-visible regions; the V-560, a double-beam, doublemonochromator instrument for the UV-visible regions; and the V-570, for the near-IR region. Jenway now has available a Model 6105, an improved version of their Model 6100, capable of operating over the 190-920-nm wavelength range. Two new instruments, UVIKON 940 and UVIKON 941, have been introduced by Kontron. Both are basic Model 930 instruments with additional accessories; the 940 has an
Table 1. Spectrophotometric Methods for Inorgank Substances
constituent 43 As Au Co(II1)
cu
Fe(II1) Ge(1V) Mo Mo(V1) NW) Ni NOzPd(I1)
material dental amalgam water alloys alloys concentrates phosphate rock mineral water concentrates cassiterites
V(V)
alloys wastewater fuel oil
Y subgroup rare earths Y(II1)
alloys
S2-
method or reagent (wavelength, nm; molar absorptivity; concentration range, pg/mLa)
cyclopentanone-thiodiglycolic acid, pH 5.0 (270; 0.02; 2.6-1 5.6)
ref
4,7-dimethyl-1,3,2-dioxaphospbepane-2-thio1-2-thione, toluene extn (420; 1.2 X lo4) triethylenetetraminehexamethylenephosphonicacid, pH 10 (256; 6.6 X 103; 0.5-13.5) phenylfluorone (0.010-0.300) 4,7-dimethyl-1,3,2-dioxaphosphepane-2-thiol-2-thione, isoamyl alc extn (514; 8.2 X lo3) 2’-hydroxyacetophenone benzoylhydrazone (390; 1.28-1 3.8) sulfochlorophenol S,extn on solid AV-17 sorbent in CI- form (660; 0.1) 2- [2- (4-methylquinolyl)azo]- 5-diethylaminophenol,CHCls extn (0 A 7 . 2 ) coupled with N,N-dimethylaniline (0.0100.200) diazotization with 7-amino-4-methylcoumarin 7-(2-pyridylazo)-5-chloro-8-hydroxyquinoline, pH 7.5 (0-10.64) acidify and measure HzS vapor (196) N-benzylbenzohydroxamicacid, HCl(5 10; 2-8) p-nitrochlorophosphonazo,HC104 (735; 6.2-9.3 X 104,0.32-1.12)
407 408 409 410 41 1 412 413 41 1 414 415 416 417 409 418 419 420
7-(2-pyridylazo)-S-chloro-8-hydroxyquinoline, pH 6.5 (0-8.89)
409
HaBr2 in EtOH (385)
7-(2-pyridylazo)~5-chlorc-8-hydroxyquinoline,50% EtOH, pH 9.5 (0-7.87) 2-(dimethylamino)-5-nitroso-1,4,5,6-tetrahydropyrimidine-4,6-dione,pH 4.8-6.8 (385; 6.3 X 10‘)
Unless specified otherwise. additional floppy disk drive and the 941 has the same additional drive plus a 40-MB hard drive and is supported by a VGA graphics color monitor. Labsystems has marketed the Uniskan 11, a manual single-beam photometer for the wavelength range 340-750 nm, and the Multiskan Plus, which can follow reaction kinetics and is capable of more sophisticated data manipulations. The Genesys 5 UV-visible spectrophotometer introduced by Milton Roy operates over the 2001100-nm wavelength range with a fixed 5-nm slit and has an eight-position cell holder for precise, automatic positioning of samples. Ocean Optics has developed a Model SlOOO miniature, single-strand, fiber-optic spectrometer that can be fitted with any of eight holographic or ruled gratings for operation in different wavelength regions. A double-grating, dual-beam, rapid-scanning instrument capable of acquiring 1000 scans/s over a 100-300-nm range has been marketed by Olis. Perkin-Elmer has introduced a flow injection analysis system for the company’s Lambda 2 automatic sampling spectrophotometer. The company has also unvei!ed the Lambda 11 UV-visible instrument that combines high quality and reliability with simplified operation and low cost. This model is based on the optical system of the Lambda 2 and is designed primarily for routine and educational applications. Secomam has also introduced a low-cost instrument, the Model S.2501, for routine applications. A software upgrade version 3.0 has been released for Shimadzu’s UV-2101PC and UV3101PC instruments. The interface is easier to use and a large number of powerful new acquisition, calculation, display, and reporting routines are included. Shimadzu has also unveiled as integrating sphere accessory for its Model UV160 spectrophotometer. A new UV-visible instrument from SLM-Aminco, the computer-controlled, double-beam DB3500, uses a menu-driven file management system and fast application setup for wavelength scanning, fixed wavelength, time drive, automatic rate assay, or programmed wavelength measurements. Spex has entered the UV-visible market with its Model 270M, a small spectrometer featuring f/4 optics with a resolution better than 0.1 nm, self-calibrating startups, 70 nm/s scanning, and charge-coupled device detectors. Unicam has announced its Model 8765, which uses the company’s 8700 series optical bench with a 386 PC operating in the Microsoft Windows environment. New variable-angle
specular reflectance and fiber-optics remote sampling accessories have been introduced for use with Varian’s Cary 4 and Cary 5 spectrophotometers. A few noncommercial spectrophotometers have been developed for specific applications including the measuring of absorption, emission, and/or excitation spectra of samples at temperatures of 10-313 K (390) and absorption spectra over the range of 200-1200 nm at temperatures of 20-100 OC (391). Other noncommercial instruments employing some new or different combination of features include a novel monolithic, chip-integrated, color spectrometer using a distributed wavelength filter component (392);a high-accuracy instrument employing highly collimated normal-incidence beam geometry to eliminate the need for polarization corrections (393);a modular instrument with a quartz fiber optical sensor for computer-controlled titrations (394);an interferometer system for ultraviolet and visible Fourier transform spectrophotometry (395, 396); a temperature-regulated isohyperbaric spectrometer for studying the effects of pressure on the role of hydration in biochemical reactions (397);and a laser photoacoustic sensor for analyte detection in aqueous solutions (398). Specialized Instruments or Components. A solid-state near-infrared Protronics process monitor from Katrina capable of measuring samples containing CH, NH, or O H groups has been introduced. The Quantum 2000 process analyzer from LT Industries can be configured for monitoring beer-blending processes, gasoline octane values, and general batch processes. NIRSystems recently introduced four new instruments: the Process Analyzer, for tracking chemical reactions and blending operations; the Rapid Resin analyzer, for identifying and quantifying polymers; the RapiTech analyzer, for routine analysis in the food processing industry; and the Rapid Content analyzer, for inspecting drugs during the manufacturing stage. The PIONIR 1024 near-infrared process analyzer from Perkin-Elmer is reported to provide simultaneously as many as 20 chemical and physical properties, such as octane numbers and boiling pints. Perstorp has introduced several singlechannel fiber-optic analyzers, the SCL 1000, SCP 1000, and SCP 2000, which are individually configured for determining the O H number of polyols, water in solvent streams, and the color of solvents. A spectrophotometer using a phase separator Analytical Chemistw, Vol. 86, No. 12, June 15, 1994
455R
Table 2. Spectrophotometric Methods for Organic Substances constituent material method or reagent (wavelength, nm; molar absorptivity; concentration range, pg/mLa)
ref
acetaminophen acebutolobHCI
42 1 422
suppositories first deriv, zero crossing method (240.1) pharmaceuticals S Z O ~oxidative ~coupling with p-N,N-dimethylphenylenediamine dihydrochloride (560; 3-36) pharmaceuticals Supracen Violet 3B, CHCI, (570) acetylspiram ycin pharmaceuticals direct, 0.1 M HCI (232; 4-24) @adrenergic blockers 2,3-dichloro- 1,4-naphthquinone, CH3CHO (650-670) tablets 1-amido-2-thiourea direct (262; 0-10) fertilizers injections amikacin Hantzsch reaction, pH 2.5 (356) amitriptyline pharmaceuticals ammonium reineckate, extn. with nitrobenzene (525; 10) ascorbic acid V(V), Chrom Azurol S, cetyltrimethylammonium bromide (590; 1.20 X 105) tablets pharmaceuticals reduction with Zn,p-phenylenediamine, FeNHd(S04)z (600; 50-250) azintamide, benzoyl metronidazole oral suspension alk hydrolysis with NaOH (276; 318) plant material brucine CHCI3 extn with Acid Fast Red A (515) pharmaceuticals EtOH, o-phenylenediamine.2HCI (490; 92-248) catecholamines tablets cephalosporins Cu(I1)-pyridine (465) pharmaceuticals reductionof Fe(II1) to Fe(II), 1,lO-phenanthroline (510; (1.16-9.68) X 103) chlorphenamine maleate 0.05 M H2SO4, second deriv (228-260) tablets chlorphenoxamineHC1 pharmaceuticals Co(II), SCN-, C6H6 extn (625) CHCI3 extn with picramic acid (500; 25-250) chlorpromazine pharmaceuticals Na2W04, pH 6 (390) chlortetracycline urine indirect, Azure A + Azure B (610; 0.02-0.4) chondroitin sulfate C pharmaceuticals Supracen Violet 3B, pH 1.3 (575) clonidine cysteine 2-chloro-1-methylpyridiniumiodide, pH 9.0 (314; 1.02 X lo4) pharmaceuticals Na2W04, pH 6 (338) doxycycline syrups ephedrine CHC13 extn, third deriv (240-340) EtOH, dual wavelength (281 and 341) fenbufen and metronidazole capsules pharmaceuticals aromatic aldehydes, acid (4-10) furanochromones hydrolysis with HzS04, dual wavelength (244 and 248; 1 mg/mL) gentamycin pharmaceuticals acetylacetone-formaldehyde reagent (415; 20-160) glutamic acid glutathione 2-chloro-1-methylpyridinium iodide, pH 9.0 (314; 1.02 X lo4) pharmaceuticals Fe(III), 1,lO-phenanthroline (512) hydrazine derivs p-nitroaniline (400; 1.7 X lo4) 3-hydroxy-4( 1H)-pyridone plant extracts imipramine pharmaceuticals ammonium reineckate, extn with nitrobenzene (525; 10) kanamycin injections Hantzsch reaction, pH 2.5 (356) tetramethyl-p-phenylenediamine,pH 5.4 (610) serum lipid hydroperoxides lysine [ Fe(CN)5N0I2- to produce pipecolic acid, Simon-Awe method for N-heterocyclic amino acids nicoumalone ~coupling with p-N,N-dimethylphenylenediaminedihydrochloride (560; 4-40) pharmaceuticals S Z O ~oxidative formation of molybdophosphoric acid, redn with malonyl dihydrazide (780; 3.55 X 104) organophosphate pesticides plant material [Fe(CN)5N0I2- to produce proline, Simon-Awe method for N-heterocyclic amino acids ornithine orphenadrine pharmaceuticals ammonium reineckate, extn with nitrobenzene (525; 10) pharmaceuticals Na2W04, pH 6 (380) oxytetracycline suppositories first deriv, zero crossing method (251 .O) phenobarbital pharmaceuticals indirect, 2,2-diphenyl-l-picrylhydrazylradical (520; 10-300) phenothiazines pharmaceuticals CHC13, bromophenol blue (410; 1-10) phenothiazines pharmaceuticals Chrome Azurol B-Be(II), N-laurylsarcosine, EDTA, pH 4.5 (620; (5-1 1) X lo4) enzymic degradation, 2-cyanoacetamide, pH 9 (274; 0-600 pmol/L) reducing polysaccharides pharmaceuticals S ~ 0 8oxidative ~coupling with p-N,N-dimethylphenylenediamine procainamideHC1 dihydrochloride (550; 2-20) procainamide pharmaceuticals Supracen Violet 3B, CHCl3 (570) promazine CHCl3 extn with picramic acid (500; 8-80) 3-methyl-2-benzothiazolinone hydrazone, HCI, FeCI3 (61 5; 1.44-5.76) tablets ranitidine-HCI sorbic acid food steam distn, Et20 extn, reextn 0.02 N NaOH, pH 11 (252-255) plant material CHC13 extn with Acid Fast Red A (515) strychnine indir with bromopyrogallol red-cetylpyridinium ion (620) surfactants (nonionic) CHC13 extn with picramic acid (500; 16-160) thioproperazine serum triglycerides serum lipase glycerokinase and phospholpyruvate kinase, 2,4-dinitrophenylhydrazine(440) verapamil pharmaceuticals Supracen Violet 3B, CHCl3 (570) tablets differential dual wavelength (324 and 334.5) vitamin Bg
423 424 425 426 427 428 429 430 43 1 432 433 434 435 436 437 438 439 440 44 1 442 439 443 444 445 446 447 442 448 449 428 427 450 45 1 422 452 45 1 428 439 42 1 453 454 455 456 422 423 438 457 458 432 459 438 460 423 46 1
Unless specified otherwise. with a porous poly(tetrafluoroethy1ene) membrane to carry out solvent extractions was has been constructed and applied to the determination of iron(III), copper, and aluminum as their 8-quinolinol complexes (399). In the area of photometric chromatography detectors, the effects of a nonlinear response of a photodiode array in the application of certain chemometric procedures for data analysis has been examined theoretically and illustrated (400). The use of absorbance measurements at pairs of wavelengths with diode-array detectors has been suggested as a way to improve sensitivity and specificity (401). Various physical designs have been evaluated for ultraviolet measurements in fused 456R
AnalyticalChemistry, Vol. 66,No. 12, June 15, 1994
silica capillaries as applied in high-performance capillary electrophoresis, capillary liquid chromatography, and hydrodynamic chromatography (402). A fiber-optic flow cell incorporating separation membranes has been constructed and evaluated for spectrophotometric and chemiluminescence measurements (403). From waveguide theory and the principle of modal continuum, a relationship has been derived between the measured absorbance and the fiber-optic tube length, the analyte concentration, and the modal absorption, scattering, and coupling coefficients (404). A new photoacoustic spectrometer has been designed for continuous monitoring of trace concentrations of gaseous
pollutants (405). Various types and designs of optically transparent electrodes used in spectroelectrochemistry have been reviewed (406).
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